The present invention relates to filament winding and tape wrapping devices. More specifically, but without limitation thereto, the present invention relates to an apparatus for imparting high tensile load to a continuous-feed linear material without damaging the material.
Filament winding of composite structures has been practiced for over 30 years. Typically, dry fibrous tows of collimated fibers (e.g., glass, carbon, etc.) are pulled from one or more spools and impregnated with a resin by passing them through a resin bath. One or more impregnated tows are wound around a mandrel at various wind angles and cured.
There are variations to this process; for example, the fiber tows may be impregnated with resin and partially cured into a laterally stiff tape before being loaded onto the filament winding machine. This allows better control of the resin impregnation process, resulting in higher quality filament wound structures. The impregnated tows may initially be consolidated into tapes as wide as 60 inches, which are subsequently slit and spooled to narrower widths for loading onto a tape wrapping (i.e., filament winding) machine.
Two examples of composite structures that are fabricated by winding impregnated tows (or partially cured tapes) around a rotating hollow mandrel are pressure vessels and drive shafts. The tape wrapping of structural columns for bridges is an example in which the wrapped structure remains stationary while the winding machine rotates around the column.
Maintaining accurate control of the tension in the filament winding process is important to obtain consistent structural performance in the finished part. The magnitude of winding tension is also important. Winding the composite tape under relatively high winding tension can help reduce entrapped air in the finished part, resulting in a higher quality product. Although residual stresses resulting from high winding tension can be detrimental to structural integrity, they can also be of tremendous structural benefit if they are properly managed.
Current winding practice imparts winding tensile stresses that are only a fraction of the strength of the wound composite material. The tensile strength of a partially cured composite tape can easily exceed 60,000 pounds per square inch (psi), and yet the winding tension is typically much less than 10,000 psi. Clearly, from the standpoint of material strength, there exists the potential to develop significantly higher wound-in residual stresses by winding the materials under much higher tension loads.
The primary reason that composite materials are not wound under very high tension may be that the advantages of high tension loads have not yet been recognized or properly understood. Therefore, special material handling hardware for winding under extremely high tension has not yet been developed.
Examples of relatively recent structural applications of composites that may benefit from high winding tension are composite-wrapped structural columns and thick-walled composite flywheels.
Special design considerations are required for developing extremely high winding tension in the winding hardware, both in the mandrels and in the power winding machinery, and particularly in the material payout apparatus and tensioning devices. The payout and tensioning devices must be strong and rigid enough to withstand the higher loads without damaging fragile composite materials that are particularly susceptible to damage from tight bending radii and pinch loads during handling. Preventing such damage becomes a critical concern when handling these materials under high tension loads.
A common method in the prior art for developing filament winding (or tape wrapping) tension is to reel the filament or tape material from a braked spool. The tension that may be developed from a braked spool is limited by the tension at which the tape was wound onto the spool. To impart high tension with a braked spool, the tape must first be wound on the spool at the desired dispensing tension, which may exceed the crush strength of the inner spool core. Even with a stronger core, underlying spooled material may become distorted and damaged by overlying material when extremely high tension is applied. Therefore a braked spool is inadequate for dispensing materials under extremely high tension loads.
Another way to impart tension to a linear continuous material is to pull the material through a series of braked rollers arranged in a serpentine configuration. FIG. 1 illustrates an example of a multiple roller, frictionally braked tensioner 10 of the prior art. A series of rollers 102 are typically linked together by a common chain drive (not shown), with at least one roller being frictionally braked. A continuous linear material 104 is pulled along rollers 102 by a takeup load P. A problem with this device is that for high values of takeup load P, several rollers are required to develop the friction that prevents continuous linear material 104 from slipping, thus increasing the size of the device.
Still another way to impart tension to a linear continuous material is to pull the material through braked nip rollers 20 in FIG. 2. This device requires sufficient friction between continuous linear material 104 and opposing nip rollers 202 to impart the required braking force to continuous linear material 104. The magnitude of the friction developed is proportional to the compressive squeeze load of nip rollers 202 against continuous linear material 104. The squeeze load is distributed lightest where nip rollers 202 first contact continuous linear material 104 and increases to a maximum where nip rollers 202 are closest together. This concentration of squeeze load at a small section of linear material 104 is called point load. The maximum tension that may be applied to linear material 104 depends on the frictional grip of nip rollers 202 holding linear material 104. The frictional grip of nip rollers 202 depends partly on the squeeze load. If the squeeze load exceeds the crush strength of continuous linear material 104, point load damage occurs.
A need therefore exists in filament and tape winding applications for a compact device to dispense continuous linear materials under high tension without damaging the materials.